Cypripedium fasciculatum (Orchidaceae) Anthesis and Fruit Set in Relationship to Diapriid Activity

Abstract

We investigated the pollination biology of a rare endemic lady's slipper orchid, Cypripedium fasciculatum, in southwestern Oregon from 1998 to 2001. In 1999, a parasitic wasp (Order Hymenoptera, Family Diapriidae, Cinetus sp.) was identified as the pollinator. Understanding the relationship between pollinator activity and floral development is imperative to successfully protect rare plants. We report here on our efforts to characterize the floral phenology of C. fasciculatum and to determine the relationship between pollinator activity and C. fasciculatum anthesis and fruit set. During the 1998—2001 period, we monitored 916 C. fasciculatum flowers in three sites for selected floral events from emergence to fruit set and monitored pollinator activity in the vicinity of the orchid sites using sticky traps. Pollinator activity and fungus gnat (sciarids and mycetophilids) activity, the host insect of the pollinator, was calculated by determining the number caught per trap per day across field sites and years. Our research characterized the floral phenology of C. fasciculatum to six distinct stages, with anthesis occurring in stage 2 and 3. Anthesis in C. fasciculatum was synchronized with peak diapriid activity as well as peak fungus gnat activity. When diapriid activity was low, then fruit set was low, although the correlation was not significant. A marginal negative correlation was found between diapriid activity and the duration of anthesis. Fungus gnats, particularly sciarids, were the major component of the insect fauna in the sites. Female diapriids were the only insects found vectoring the pollinia of C. fasciculatum. We believe the non-rewarding C. fasciculatum flowers emit odors that mimic the odor of fungus gnats luring female diapriids to enter a flower in search of a host to parasitize. Trapped diapriids encounter the stigma of C. fasciculatum en route to the small posterior opening in the labellum where the pollinia is deposited as the diapriid exits.